1. Field of the Invention
The present invention relates to an electrophotographic photoreceptor. In addition, the present invention also relates to an image forming apparatus and a process cartridge using the electrophotographic photoreceptor.
2. Discussion of the Background
Electrophotography is one form of image forming method and typically includes the following processes:
(1) charging a photoreceptor in a dark place (charging process);
(2) irradiating the charged photoreceptor with light containing image information to selectively decay the charge on a lighted area of the photoreceptor, resulting in formation of an electrostatic latent image thereon (light irradiating process);
(3) developing the electrostatic latent image with a developer including a toner comprising a colorant and a binder to form a toner image on the photoreceptor (developing process);
(4) optionally transferring the toner image to an intermediate transfer medium (first transfer process);
(5) transferring the toner image, either directly or from the intermediate transfer medium, onto a receiving material such as a receiving paper ((second) transfer process);
(6) heating the toner image to fix the toner image on the receiving material (fixing process); and
(7) cleaning the surface of the photoreceptor (cleaning process).
In such image forming methods, requisites (i.e., required electrophotographic properties) for the photoreceptors are as follows:
(1) to be able to be charged so as to have a proper potential in a dark place;
(2) to have a high charge retainability (i.e., to keep the charge well in a dark place); and
(3) to rapidly decay the charge thereon upon application of light thereto (i.e., the potential of a lighted-area is low).
Until now, photoreceptors in which one of the following photosensitive layers is formed on an electroconductive substrate have been used:
(1) layers mainly including selenium or a selenium alloy;
(2) layers in which an inorganic photoconductive material such as zinc oxide or cadmium sulfide is dispersed in a binder resin;
(3) layers using an organic photoconductive material such as azo pigments and combinations of poly-N-vinylcarbazole and trinitrofluorenone; and
(4) layers using amorphous silicon.
Currently, organic photoreceptors using an organic photosensitive material are widely used because of satisfying such requisites as mentioned above and having the following advantages over the other photoreceptors:
(1) manufacturing costs are relatively low;
(2) having good designing flexibility (i.e., it is easy to design a photoreceptor having a desired property); and
(3) hardly causing environmental pollution.
As for the organic photoreceptors, the following photosensitive layers are known:
(1) a photosensitive layer including a photoconductive resin such as polyvinyl carbazole (PVK) or the like material;
(2) a charge transfer photosensitive layer including a charge transfer complex such as a combination of polyvinyl carbazole (PVK) and 2,4,7-trinitrofluorenone (TNF) or the like material;
(3) a photosensitive layer in which a pigment, such as phthalocyanine or the like, is dispersed in a binder resin; and
(4) a functionally-separated photosensitive layer including a charge generation material (hereinafter a CGM) and a charge transport material (hereinafter a CTM).
Among these organic photoreceptors, the photoreceptors having a functionally-separated photosensitive layer especially attract attention now.
The mechanism of forming an electrostatic latent image in the functionally-separated photosensitive layer having a charge generation layer (hereinafter a CGL) and a charge transport layer (hereinafter a CTL) formed on the CGL is as follows:
(1) when the photosensitive layer is exposed to light after being charged, light passes through the light-transmissive CTL and then reaches the CGL;
(2) the CGM included in the CGL absorbs the light and generates a charge carrier such as an electron and a positive hole;
(3) the charge carrier is injected to the CTL and transported through the CTL due to the electric field formed by the charge on the photosensitive layer;
(4) the charge carrier finally reaches the surface of the photosensitive layer and neutralizes the charge thereon, resulting in formation of an electrostatic latent image.
For such functionally-separated photoreceptors, a combination of a CTM mainly absorbing ultraviolet light and a CGM mainly absorbing visible light is effective and is typically used. Thus, functionally-separated photoreceptors satisfying the requisites as mentioned above can be prepared.
Currently, needs such as high speed recording and downsizing are growing for electrophotographic image forming apparatus. Therefore, an increasing need exists for durable photoreceptors having high reliability, which can produce good images even when repeatedly used for a long period of time while having the above-mentioned requisites.
Photoreceptors used for electrophotography receive various mechanical and chemical stresses. When a photoreceptor is abraded due to these stresses and its photosensitive layer is thinned, undesired images are produced.
In attempting to solve this abrasion problem, a technique in which a filler is included in a photoreceptor, and a technique in which a filler is dispersed in a surface of a photosensitive layer have been disclosed in Japanese Laid-Open Patent Publications Nos. (hereinafter JOPs) 1-205171, 7-333881, 8-15887, 8-123053 and 8-146641.
The durability, the ability to produce high quality images, and the stability of a photoreceptor including a filler depend on the dispersing condition of the filler in the photoreceptor.
When the filler is unevenly dispersed or large aggregations thereof exist in a layer (such as a photosensitive layer and a protective layer), the transparency of the layer decreases and irradiated light is scattered by such filler. As a result, a charge is unevenly generated and thereby the resultant image quality decreases. In addition, when a charge generated in the photosensitive layer is transported to the surface of the photoreceptor, the filler interferes with the charge transportation. As a result, the surface of the photoreceptor has an uneven potential and thereby the resultant image quality decreases. Further, there are problems such that cleanability of the photoreceptor deteriorates and that the cleaning blade becomes chipped, when large aggregations of the filler exist on the surface of the photoreceptor.
These problems can be solved by preparing a layer coating liquid in which a filler is well dispersed and aggregations thereof hardly exist. Such a layer coating liquid also needs to have high dispersion stability of the filler. When the dispersion stability of the filler is poor, the filler tends to precipitate at a time not only the layer coating liquid is preserved for a long period of time but also subjected to the preparation of the resultant layer. Such a layer coating liquid cannot stably prepare a photoreceptor.
In attempting to improve the dispersibility (i.e., dispersion and aggregation state) of a filler in a layer and the dispersion stability of a filler in a layer coating liquid, a technique in which a dispersing agent is added to a layer coating liquid have been disclosed in JOPs 2003-149849 and 2002-268257.
The polar surface of the filler is modified with the dispersing agent disclosed therein. The modified filler has better affinity for a solvent or a resin in the layer coating liquid, and thereby the dispersibility and dispersion stability of the filler in the layer coating liquid improve. It is described therein that the layer coating liquid including the dispersing agent can prepare a photoreceptor in which the filler is well dispersed in the layer, which has good abrasion resistance and is capable of producing images having good properties.
JOP 2006-63341 discloses an inorganic material surface-grafted with a charge transport moiety. It is disclosed therein that an imaging member including such a surface-grafted inorganic material as a filler has good abrasion resistance. The charge transport moiety is grafted to the surface of the inorganic material via a linking group or an anchoring group. It seems that there is a difficulty in applying this method to a variety of materials.
As mentioned above, by adding a filler to the outermost layer of a photoreceptor, durability of the photoreceptor increases. However, mechanical durability thereof is yet lower than that of a photoreceptor using an amorphous silicon. Since the needs such as high speed recording and downsizing may grow much more in the future, the need for a much more durable organic photoreceptor is increased.